The known ester compounds of the type encompassed by the terminology used in the title, such as those of 1-phenylvinylphosphonic acid, are useful as intermediates for further chemical synthesis of, for example, plasticizers, flame retardants, and the like. Such compounds are also useful as comonomers, in minor amount, with a predominant amount of other vinylic monomers to impart flame retardancy and to alter the surface characteristics of the resulting polymer product. Various disclosures exist in regard to the process of formation of such compounds.
U.S. Pat. No. 2,365,466 to L. A. Hamilton describes their formation by reaction of an alcohol or phenol with a phosphonic acid dichloride.
British Patent No. 974,988 describes the formation of alpha, beta-unsaturated phosphonic acid esters, including certain 1-phenylvinyl phosphonic acid dialkyl ester compounds, by the reaction of an xcex1-hydroxyphosphonic acid ester with thionyl chloride.
U.S. Abramov in Zh. Obshch. Khim, 22, pp. 709-712 (1952) discloses the reaction of aldehydes and ketones upon the salts of dialkyl phosphorous acids to form diesters of alpha hydroxyalkylphosphonic acids.
More recently, M. Yamashita et al. in Bull. Chem. Soc. Japan, 53, 1625-1628 (1980) described the Arbuzov reaction of trimethyl and triethyl phosphites with acyl chlorides to give 1-oxaalkylphosphonates which could be converted to vinylphosphonates by the Wittig reaction using methylenetriphenylphosphorane.
T. Hirao et al. in Bull. Chem. Soc. Japan, 55, 909-913 (1982) describe the palladium catalyzed reaction of aryl bromides with dialkyl phosphites in the presence of triethylamine to form dialkyl arylphosphonates and the similar treatment of vinyl bromides to give dialkyl vinylphosphonates.
In its broadest embodiment the present invention relates to the formation of hydrocarbylvinyl phosphonic acid hydrocarbyl esters which can be represented by the formula 
where any of R1 through R5 can independently be hydrogen, alkyl or aryl. Groups R1 and R2 can be joined together in a single cyclic structure bonded at each end thereof to the oxygens attached to the phosphorus atom (such as when neopentylene glycol is used as a reactant as described and claimed in U.S. Ser. No. 386,393, filed on even date herewith, and entitled xe2x80x9cNeopentylene Phosphonate Compoundsxe2x80x9d).
The present invention can be defined as a process for the synthesis of a hydrocarbylvinylphosphonic acid hydrocarbyl ester which comprises: (a) the base-catalyzed addition of a dihydrocarbyl phosphite to an aldehyde or ketone, such as acetophenone, to form a dihydrocarbyl 1-hydroxy hydrocarbylphosphonate compound, such as a dialkyl 1-hydroxy-1-phenylethyl-phosphonate; (b) the acid-catalyzed esterification, e.g., acetylation, of the compound from (a) with an acid anhydride, such as acetic anhydride, to form an esterified, e.g., acylated intermediate; and (c) the elimination of acid, e.g., by deacetylation, from the intermediate from (b) to form the desired hydrocarbylvinylphosphonic acid hydrocarbyl ester, such as a 1-phenylvinylphosphonic acid dialkyl ester.
Another embodiment of the present invention involves step (c) of the forgoing process in which carboxylic acid is removed from the intermediate to form the desired dialkyl ester product.
Still another embodiment of this invention is the base catalyzed reaction (a) in which an aldehyde or ketone and dihydrocarbyl phosphite are reacted using a nonnucleophilic strong organic base as catalyst.
The initial step in the process of the present invention comprises the reaction of a dialkyl phosphite of the formula 
where R1 and R2 are each as defined above with both, preferably, being an unsubstituted or substituted alkyl group of no more than eight carbon atoms, e.g., most preferably from C1 to C4 alkyl, with an aldehyde or ketone of the formula 
R3, R4, and R5 being defined as set forth above, acetophenone being a preferred ketone, to form a dihydrocarbyl 1-hydroxy-hydrocarbylphosphonate of the formula 
where each of the xe2x80x9cRxe2x80x9d groups depicted in the preceding formula is as defined before above. This step is practiced with base catalysis using, as a preferred class of catalyst, a nonnucleophilic strong organic base (such as one where the pKa of the conjugate acid is greater than about 10), such as 1,1,3,3-tetramethylguanidine. If desired, an alkali metal alkoxide catalyst (such as sodium ethoxide in ethanol solvent) in a suitable hydrocarbon solvent, such as hexane may also be used but it is less preferred. The reaction can be carried out at slightly below to slightly above ambient temperature (e.g., from 0xc2x0 C. to 50xc2x0 C.) with the desired product being recovered by recrystallization. The compound depicted above carries the following essential structure 
which allows for eventual elimination of the elements of water to form a phosphorus-substituted alkene.
The type of substituents on any of the xe2x80x9cRxe2x80x9d groups shown in the preceding formula can be selected from halo (such as chloro or bromo), if desired. The type of substituents that are contemplated for use on the phenyl group, if such is selected, for example as R3, can be selected from halo (bromo or chloro), hydroxyl and alkyl.
The next step in the process of the invention involves the acid-catalyzed acetylation of the compound produced in the preceding step by its reaction with a suitable acid anhydride, such as acetic anhydride, to form an intermediate of the formula 
where the three xe2x80x9cRxe2x80x9d groups have the meanings given above, and A is derived from the acid anhydride used. In the case of acetic anhydride, A will be methyl. This reaction is preferably conducted at temperatures in the range of from about 10xc2x0 C. to about 50xc2x0 C., preferably using a strong acid (for example, one having a pKa of below about 1) as a catalyst. Such acid catalysts as a perhalic acid catalyst or a strong mineral acid, such as sulfuric acid can be used. The term xe2x80x9cperhalic acidxe2x80x9d is intended to encompass such acidic catalysts as perchloric acid and perbromic acid. Trifluoromethanesulfonic acid is another example of a catalyst that can be employed. Other acid catalysts which can be used include sulfonated macroreticular ion exchange resins (e.g, AMBERLYST 15, 35, or 36 brand resins), perfluorinated ion exchange powders (NAFION brand ion exchange materials), or acidic clays.
The intermediate from the previously described step, which, in a preferred embodiment, is a (1-dialkylphosphono-1-phenylalkyl carboxylate, for example, a (1-dialkylphosphono-1-phenylethyl acetate, is then converted to the desired 1-phenylvinylphosphonic acid dialkyl ester compound by deacetylation of the intermediate, preferably by heating at temperatures in the range of from about 50xc2x0 C. to about 215xc2x0 C. under reduced or atmospheric pressure (e.g., at about 50 to about 70 mm Hg pressure). The presence of a catalyst in the reaction medium will allow for the use of lower temperatures. Generally speaking, the use of higher temperatures within the above-described range will tend to give lower yields and poorer selectivities than the use of lower temperatures. At lower temperatures (e.g., at 50xc2x0 C. to 125xc2x0 C.), solid superacid catalysts (e.g., materials available under the trademarks AMBERLYST and NAFION) can be used to produce higher yields and higher selectivities of product than are achievable at the previously discussed higher reaction temperatures.